Migrate to GitHub actions?

[adbancroft]

I'd like to fix the AVR CI, but don't have an Appveyor account. I realize I could join Appveyor, but it seems like the better solution might be to migrate to GitHub actions for CI.

Is there interest in this? If so, I can create a PR.

[kimwalisch]

Yes, please.

[kimwalisch]

@adbancroft Thanks for your work! I wanted to have automated testing for many years since many users are too lazy to manually test their changes extensively before submitting pull requests. Often pull requests would only work on their PC's default operating system and compiler...